<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
  </style>
</head>
<body>
  <div class="header">test</div>
  <h2 id="test" class="header">qwer</h2>
  <h2 id="test" class="header">qwer</h2>
  <h2 id="test" class="header">qwer</h2>
  <script>
    var box = document.querySelector('h2.header')
    console.log(box)

    // var list = document.querySelectorAll('.header')
    // console.log( list )

  </script>
</body>
</html>